Ion channels control a wide range of cellular activities in both excitable and non-excitable cells (Hille, 2002). Ion channels are attractive therapeutic targets due to their involvement in many physiological processes. In excitable cells, the coordinated function of the resident set of ion channels controls the electrical behavior of the cell. Plasma membrane calcium channels are members of a diverse superfamily of voltage gated channel proteins. Calcium channels are membrane-spanning, multi-subunit proteins that allow controlled entry of Ca2+ ions into cells from the extracellular fluid. Excitable cells throughout the animal kingdom, and at least some bacterial, fungal and plant cells, possess one or more types of calcium channel. Nearly all “excitable” cells in animals, such as neurons of the central nervous system (CNS), peripheral nerve cells and muscle cells, including those of skeletal muscles, cardiac muscles, and venous and arterial smooth muscles, have voltage-gated calcium channels. Voltage-gated calcium channels provide an important link between electrical activity at the plasma membrane and cell activities that are dependent on intracellular calcium, including muscle contraction, neurotransmitter release, hormone secretion and gene expression. Voltage-gated calcium channels serve to integrate and transduce plasma membrane electrical activity into changes in intracellular calcium concentration, and can do this on a rapid time scale.
Multiple types of calcium channels have been identified in mammalian cells from various tissues, including skeletal muscle, cardiac muscle, lung, smooth muscle and brain. A major family of this type is the L-type calcium channels, which include Cav1.1, Cav1.2, Cav1.3, and Cav1.4, whose function is inhibited by the familiar classes of calcium channel blockers (dihydropyridines such as nifedipine, phenylalkylamines such as verapamil, and benzothiazepines such as diltiazem). Additional classes of plasma membrane calcium channels are referred to as T (Cav3.1 and Cav3.2), N (Cav2.2), P/Q (Cav2.1) and R (Cav2.3). The “T-type” (or “low voltage-activated”) calcium channels are so named because they open for a shorter duration (T=transient) than the longer (L=long−lasting) openings of the L-type calcium channels. The L, N, P and Q-type channels activate at more positive potentials (high voltage activated) and display diverse kinetics and voltage-dependent properties.
Because of the crucial role in cell physiology, modulation of calcium channel activity can have profound effects. Mutations in calcium channel subunits have been implicated in a number of genetic diseases including familial hemiplegic migraine, spinocerebellar ataxia, Timothy Syndrome, incomplete congenital stationary night blindness and familial hypokalemic periodic paralysis. Modulation of voltage-gated calcium channels by signaling pathways, including c-AMP-dependent protein kinases and G proteins is an important component of signaling by hormones and neurotransmitters (Catterall, 2000). Pharmacological modulation of calcium channels can have significant therapeutic effects, including the use of L-type calcium channel (Cav1.2) blockers in the treatment of hypertension (Hockerman, et al., 1997) and more recently, use of Ziconotide, a peptide blocker of N-type calcium channels (Cav2.2), for the treatment of intractable pain (Staats, et al., 2004). Zicontide is derived from Conotoxin, a peptide toxin isolated from cone snail venom, must be applied by intrathecal injection to allow its access to a site of action in the spinal cord and to minimize exposure to channels in the autonomic nervous system that are involved in regulating cardiovascular function. Ziconotide has also been shown to highly effective as a neuroprotective agent in rat models of global and focal ischemia (Colburne et. Al., Stroke (1999) 30, 662-668) suggesting that modulation of N-type calcium channels (Cav2.2) has implication in the treatment of stroke.
Clinical and preclinical experiments with ziconotide and related peptides confirm a key role of N-type calcium channels in transmitting nociceptive signals into the spinal cord. Identification of N-type calcium channel blockers that can be administered systemically, and effectively block N-type calcium channels in the nociceptive signaling pathway, while sparing N-type calcium channel function in the periphery would provide important new tools for treating some forms of pain. The present invention describes blockers of N-type calcium channels (Cav2.2) that display functional selectivity by blocking N-type calcium channel activity needed to maintain pathological nociceptive signaling, while exhibiting a lesser potency at blocking N-type calcium channels involved in maintaining normal cardiovascular function. See WO2007085357, and WO2007028638.
There are three subtypes of T-type calcium channels that have been identified from various warm blooded animals including rat [J. Biol. Chem. 276(6) 3999-4011 (2001); Eur J Neurosci 11(12):4171-8 (1999); reviewed in Cell Mol Life Sci 56(7-8):660-9 (1999)]. These subtypes are termed α1G, α1H, and α1I, and the molecular properties of these channels demonstrate 60-70% homology in the amino acid sequences. The electrophysiological characterization of these individual subtypes has revealed differences in their voltage-dependent activation, inactivation, deactivation and steady-state inactivation levels and their selectivity to various ions such as barium (J Biol. Chem. 276(6) 3999-4011 (2001)). Pharmacologically, these subtypes have shown differing sensitivities to blockade by ionic nickel. These channel subtypes are also expressed in various forms due to their ability to undergo various splicing events during their assembly (J Biol. Chem. 276 (6) 3999-4011 (2001)).
T-type calcium channels have been implicated in pathologies related to various diseases and disorders, including epilepsy, essential tremor, pain, neuropathic pain, schizophrenia, Parkinson's disease, depression, anxiety, sleep disorders, sleep disturbances, psychosis, schizophrenia, cardiac arrhythmia, hypertension, pain, cancer, diabetes, infertility and sexual dysfunction (J Neuroscience, 14, 5485 (1994); Drugs Future 30(6), 573-580 (2005); EMBO J, 24, 315-324 (2005); Drug Discovery Today, 11, 5/6, 245-253 (2006)). See also patent and publications US2007/0105820, U.S. Pat. No. 6,462,032, U.S. Pat. No. 7,084,168, U.S. Pat. No. 6,608,068, U.S. Pat. No. 7,253,203, WO86/03749, WO91/06545, WO91/04974, US2006/0258659, US2006/0252812, US2006/0252758, Fensome et al., Bioorg. Med. Chem. Lett. 12, 3487-3490 (2002), and Andreani et al, Acta Pharm Nord., 2(6), 407-414 (1990). See also simultaneously filed application referred to as Ser. No. 60/997,705, herein incorporated by reference in its entirety.